FIELD OF THE INVENTION
This invention relates to a polarization converting apparatus and an optical instrument such as a projector having the same.
There is known a polarization converting apparatus for converting non-polarized light into rectilinearly polarized light, circularly polarized light or the like while minimizing the loss of the quantity of light.
Such polarization converting apparatus is disclosed in the applicant's U.S. Pat. No. 4,560,999, U.S. Pat. No. 4,989,076, and U.S. Pat. No. 4,864,390, Japanese Laid-Open Pat. Application No. 61-90584 etc. The apparatus disclosed in these publications is such that light from a light source comprising an arrangement of a lamp and a mirror and/or a lens is divided by a polarizing beam splitter into two polarized beams whose planes of polarization are orthogonal to each other, whereafter the plane of polarization of one of the polarized beams is rotated so that the plane of polarization of said one polarized beam may coincide with the plane of polarization of the other polarized beam, and in this manner, the non-polarized light-from the light source is efficiently converted into rectilinearly polarized light.
However, in this apparatus, the polarizing beam splitter comprises a cube comprising square plates or prisms stuck together and therefore, the light incidence surface thereof is rectangular and part of a light beam usually of circular cross-sectional shape from the light source has been eclipsed by the light incidence surface. The quantity of the light eclipsed by this light incidence surface is not negligibly small. Accordingly, the prior-art polarization converting apparatus could not be said to achieve its purpose sufficiently.